A coffee machine may be fitted with a milk frothing system for preparing coffee types that include steamed-milk foam, such as for example a cappuccino or a caffèlatte. The general purpose of the milk frothing system in such a machine is to prepare or aid in preparing a milk foam comprised of tiny (preferably micro-size) bubbles and having a temperature in the range of about 50-70° C.
Known milk frothing systems may froth the milk by mixing steam, air and milk by means of a Venturi-pipe. Steam is then forced through the Venturi-pipe, creating an underpressure therein that is used to suck in both atmospheric air and milk through an air supply line and a milk supply line, respectively. The amount of milk and air that is sucked in is controlled via dosing restrictions in the respective supply lines. This approach is favorable from the point of view of manufacturing costs as it may require only one, typically electrically powered, pump for providing a steam generator with water to be vaporized, and to keep the system properly pressurized. Unfortunately, however, the mixing process within the Venturi-pipe may be somewhat violent and erratic, as a result of which milk residue may unintentionally end up in and further narrow the dosing restriction in the air supply line. Eventually, the air dosing restriction may even get blocked altogether, in which case the milk frothing system will stop functioning.
An alternative to sucking atmospheric air into a steam driven Venturi-pipe is to force compressed air into the steam line leading up to the Venturi, which Venturi may then be used exclusively to suck in milk. The use of compressed air may do away with the need for a vulnerable air dosing restriction, since a separate air pump or compressor may be used to both compress and meter the supply of air. An extra pump, however, would also raise the manufacturing costs of a milk frothing system, and consequently render this alternative less advantageous.
It is an object of the present invention to overcome or mitigate the aforesaid disadvantages of known milk frothing systems, and to provide an economically manufacturable milk frothing system in which the risk of congestion of an air supply line by milk residue is minimized.